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From seabirds to Arctic mammals, chimpanzees to orangutans, various animals engage in mouth-to-mouth contact. Now, scientists suggest that Neanderthals did it too – and might even have locked lips with early Homo sapiens.

Shared Microbial Evidence

This isn't the initial instance scientists have suggested ancient relatives and Homo sapiens were closely connected. In earlier research, scientists have found modern people and their Neanderthal relatives possessed the same mouth microbe for millions of years after the two species split, implying they swapped saliva.

"Likely they were kissing," the researcher noted, explaining that the idea chimed with research that has found people of certain genetic backgrounds have bits of Neanderthal DNA in their genetic makeup, revealing genetic mixing was at play.

Romantic Interpretation

"It certainly puts a more romantic spin on ancient interactions," the lead researcher said.

Publishing in the journal Evolution and Human Behavior, Brindle and her team detail how, to investigate the historical roots of kissing, they first had to come up with a definition that was not limited to how people kiss.

Defining Kissing

"There have been some previous attempts to define a intimate act, but it's largely focused on humans, which implies that basically other animals do not engage in this. Now we understand that they probably do, it might just not look from what human kissing resembles," said the evolutionary biologist.

However, she said some behaviors that looked like intimate contact were something rather different – such as the processing and food sharing, or "kiss-fighting", seen in fish known as French grunts.

As a result the team came up with a definition of kissing centered around social behaviors involving directed mouth-to-mouth contact with a member of the identical group, with some movement of the oral area but no transfer of food.

Research Approach

Brindle said they focused on reports of kissing in non-human species from the African continent and Asia, including bonobos, apes and great apes, and employed digital recordings to confirm the observations.

Scientists then combined this information with details on the genetic connections between living and ancient species of such animals.

Evolutionary Origins

Researchers propose the findings suggest intimate contact evolved somewhere between 21.5m and 16.9 million years ago in the ancestors of the large apes.

Placement of ancient hominins on this evolutionary lineage means it is probable they, too, engaged in a intimate act, the scientists say. But the behavior may not have been confined to their own species.

"The fact that humans engage intimately, the reality that we now have shown that ancient relatives probably engaged, suggests that the two [species] are also likely to have engage," the researcher noted.

Biological Importance

Although the evolutionary explanation is discussed, the expert said intimate contact could be used in reproductive situations to possibly increase mating outcomes or help choose between partners, while it might help reinforce bonding when practiced in a platonic way.

Another expert in the behavior of primates commented that as intimate contact was observed in a broad spectrum of primates it was logical its roots lie deep in our ancient history, and an examination of different forms of kissing among a broader range of animals might push its origins back further still.

"Behaviors that we consider as signatures of our species, like intimate contact, are not exclusive to us if we examine carefully at different species," the expert noted.

Cultural Aspects

Another professor explained that intimate contact had a cultural element as it was not universal to all human groups.

"However, as humans we thrive or fail on the strength of our emotional bonds, and ways of promoting trust and intimacy will have been important for eons," she said. "It might be an concept that appears a bit incongruous to our misplaced ideas of a supposedly aggressive and ancient history, but actually it should be no surprise that ancient hominins – and even them and our human ancestors collectively – engaged intimately."